1. Field of the Invention
The present invention relates to a signal selecting circuit for use with a satellite broadcasting reception converter installed in the outdoors or the like.
2. Description of the Related Art
A conventional signal selecting circuit will be described with reference to FIG. 3. A first reception signal (e.g. vertically-polarized satellite broadcasting signal) and a second reception signal (e.g. horizontally-polarized satellite broadcasting signal) are inputted to a first input terminal 31 and a second input terminal 32. The first reception signal is amplified by a first high-frequency amplifier 33, and the second reception signal is amplified by a second high-frequency amplifier 34. The first high-frequency amplifier 33 and a common output terminal 36; and the second high-frequency amplifier 34 and the common output terminal 36 are connected by a first microstrip line 35 and a second microstrip line 37, respectively. The first reception signal amplified by the first high-frequency amplifier 33 is outputted through the first microstrip line 35 to the common output terminal 36, and the second reception signal amplified by the second high-frequency amplifier 34 is outputted through the second microstrip line 37 to the common output terminal 36.
The first microstrip line 35 and the second microstrip line 37 have a predetermined characteristic impedance, and the lengths thereof are set to 1/2 wavelength of frequencies of the first reception signal and the second reception signal which are respectively transmitted through the first microstrip line 35 and the second microstrip line 37.
A DC voltage B is supplied through a switch 38 to the first high-frequency amplifier 33 or the second high-frequency amplifier 34. That is, when the first reception signal is received, the switch 38 allows the DC voltage B to be supplied to the first high-frequency amplifier 33 to set the first high-frequency amplifier 33 in the operable state, whereby the first reception signal inputted to the first input terminal 31 is amplified by the first high-frequency amplifier 33 and then supplied through the first microstrip line 35 to the common output terminal 36. At that time, the second high-frequency amplifier 34 is de-energized by a low DC voltage applied thereto through a resistor 40. As a consequence, the second reception signal inputted to the second input terminal 32 is not amplified but attenuated by the second high-frequency amplifier 34. Moreover, since the length of the second microstrip line 37 is set to the 1/2 wavelength, the impedance of the second microstrip line 37 increases as seen from the common output terminal 36, and hence the second reception signal is not delivered to the common output terminal 36. Accordingly, only the first reception signal is inputted to the subsequent amplifier 41.
Then, since the low DC voltage is applied to the second high-frequency amplifier 34, its output impedance is fixed so that the input impedance of the subsequent amplifier 41 becomes difficult to be affected.
On the other hand, when the second reception signal is received, the switch 38 allows the DC voltage B to be supplied to the second high-frequency amplifier 34 to set the second high-frequency amplifier 33 to the operable state, whereby the second reception signal inputted to the second input terminal 32 is amplified by the second high-frequency amplifier 33 and then supplied through the second microstrip line 37 to the common output terminal 36. At that time, the first high-frequency amplifier 33 is de-energized by the low DC voltage applied thereto through a resistor 39. As a consequence, the first reception signal inputted to the first input terminal 31 is not amplified but attenuated by the first high-frequency amplifier 33. Also, since the length of the first microstrip line 35 is set to the 1/2 wavelength, the impedance of the first microstrip line 35 increases as seen from the common output terminal 36, and hence the first reception signal is not delivered to the common output terminal 36. Accordingly, only the second reception signal is inputted to the subsequent amplifier 41.
Then, also in this case, since the low DC voltage is applied to the first high-frequency amplifier 33, its output impedance is fixed so that the input impedance of the subsequent amplifier 41 becomes difficult to be affected.
In the above-mentioned conventional signal selecting circuit, although neither a reception signal nor a disturbance signal is outputted to the common output terminal 36 from the line through which an undesired reception signal is transmitted, a disturbance signal such as an image signal relative to a desired reception signal is outputted to the common output terminal 36 from the line through which the desired reception signal is transmitted. There is then the risk that a disturbance will occur.